1. Field of the Invention
The present invention relates to a receiver and receiving method for spread spectrum signals, and more particularly, is preferably applied to a radio communication system such as a portable telephone system.
2. Description of the Related Art
In a cellular radio communication system, an area of providing a communication service is divided into cells of desired size and every base station serving as a fixed radio station is installed in a cell. A portable telephone as a mobile radio station radio-communicates with the base station in the cell in which the portable telephone itself is located.
There have been proposed various kinds of systems as a communication system between the portable telephone and the base station. One of the systems is a code division multiple access (CDMA) system.
In The CDMA system, a pseudo noise sequence (PN) code having a characteristic pattern composed of a pseudo random number series code is assigned to each of communication lines in a transmitting side. The assigned PN code is multiplied by a primary modulation signal of the same carrier frequency, so that the PN code is spread to a band wider than an original frequency band (this is called a spectrum diffusion hereinafter) and a secondary modulation signal subjected to the spectrum diffusion is transmitted.
On the other hand, a receiving side, receiving a transmitted signal sent from the transmitting side, multiplies the received signal by the PN code having the same series pattern and the same phase as those of the PN code assigned to each of the communication lines in the transmitting side, so that the received signal is subjected to an inverse diffusion process to obtain a primary modulation output. In addition, the primary modulation output is demodulated so as to restore transmitted data.
As described above, according to the CDMA system, the same PN code is previously set to be mutually generated in the transmitting side and the receiving side. In the receiving side, only when the inverse diffusion process is carried out by using the PN code having the same series pattern and the same phase as those of the PN code used in the transmitting side, the primary modulation output can be demodulated, and therefore, an excellent secrecy can be advantageously achieved.
Further, in a cellular radio communication system utilizing the CDMA system, the base station of the transmitting side repeatedly transmits pilot signals generated by diffusing data comprising all xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d with the PN code in order to get a synchronization, to track a synchronization (tracking) and to reproduce a clock in a mobile station side. The mobile station of the receiving side first receives the pilot signals constantly sent from the base stations at the time of turning power on.
The mobile station of the receiving side receives a plurality of pilot signals transmitted in a multipass through a plurality of transmission paths in a superposed state to obtain receiving timings different from each other for the respective pilot signals. Then, when an actual talking is started, the mobile station multiplies the received signals in the multipass (referred as multipass signals, hereinafter) which are received in a delayed state with a plurality of demodulators provided therein by the respective PN codes having the phases corresponding to the obtained receiving timings in order to perform an inverse diffusion process. A combiner combines the plurality of inverse-spread received signals in a state in which their timings are set to the same timing. Thus, a signal-to-noise power ratio (SN ratio) of the demodulation signal is improved. That is, the mobile station is designed to constitute a Rake receiver which prevents direct waves in a multipass and reflection waves from interfering with each other so as to lower electric power.
For instance, in the CDMA system which has been already standardized in U.S.A. as an IS-95 system, as illustrated in FIG. 1, a mobile station 1 of the receiving side receives pilot signals P1 to P3 in a multipass which are sent from an object base station 2 with a time delay due to the reflection of buildings 3 and 4. Here, the pilot signals P1 to P3, which are received by the mobile station 1, have the same series pattern, however, have different phase shifts due to the time delay.
The mobile station 1 calculates the correlation values of the pilot signals P1 to P3 and the respective PN codes generated therein with a circuit called a searcher (not shown) provided therein while moving phases of the PN codes, so that the phases of the pilot signals P1 to P3 are detected. Then, the mobile station 1 synchronizes them with a system clock cn to all the base station and mobile stations in a whole system with the pilot signal P1 having the largest correlation value as a reference. Thus, the pilot signals P1 to P3 are composed of the PN codes whose cycles have the same series pattern of 32768 series (215), and they have a common position of a phase xe2x80x9c0xe2x80x9d and the phases of the pilot signals are shifted respectively by several tens of chips.
In the pilot signals P1 to P3, the direct wave is the largest correlation value. As the arrival time of the reflection wave is late, the correlation value of the reflection wave becomes smaller than that of the direct wave. This is associated with the phase shift representing the difference of arrival time among the pilot signals P1 to P3 which reach the mobile station. As a matter of course, the pilot signal P1 being the direct wave, which has the largest correlation value, has the smallest phase.
When the mobile station starts an actual talking after exchanging control data including such pilot signals P1 to P3 with the base station, it initially detects received signals (multipass signals) S1, S2 and S3 in a multipass state by calculating the correlation values with a searcher, as shown in FIG. 2.
Then, when the mobile station detects the multipass signals S1 to S3, the multipass signal S1 having the largest correlation value among them is used as a reference signal. The mobile station periodically detects (referred to as a steady search, hereinafter) with the searcher whether or not the multipass signals S1 to S3 exist within an arbitrary search window range called a xe2x80x9csearch windowxe2x80x9d having the phase position 64 of the reference signal at a center which is designated by the base station. When the mobile station can detect the multipass signals S1 to S3, it demodulates transmitted data by employing the multipass signals S1 to S3 (phase positions 64, 68 and 70) which are the three highest signal strength.
Here, the xe2x80x9csearch windowxe2x80x9d is determined to range between xc2x120 of the multipass signal S1 serving as the reference signal S1 (from 44 to 84 in the phase position). The mobile station always carries out the steady search within the above-described search window range even during receiving of real data.
Since it is generally difficult to consider that there exists an extremely big time difference between the multipass signal S1 at the phase position 64 which arrives at the mobile station first, and the multipass signals S2 and S3 which arrive with a delay due to the reflection of buildings, in this case, a search window range is determined so as to detect the multipass signals S1 to S3, which are the three highest signal strength, by searching the phase range between xc2x120 of the multipass signal S1 at the phase position 64, which arrives first and is served as the center.
As described above, according to the CDMA system of the IS-95 system, the multipass signal S1 at the phase position 64, which arrives at the mobile station first, is used as the reference signal, and the time management such as the acquisition of synchronization, the tracking of synchronization, the reproduction of clocks, etc. is performed based on the reference signal.
However, in the case where the reference signal on lost due to the change of a transmission condition, the mobile station quickly uses the multipass signal S2 at the phase position 68, which arrives at the mobile station next, as a reference signal in accordance with a prescribed time constant in order to perform time management.
In order to perform such time management, the mobile station uses a counter (called a system time counter, hereinafter) on which the time management is based. This system time counter always follows a reference signal. It the reference signal is lost due to the change of a transmission condition, as a matter of course, the system time counter follows a next reference signal to carry out the time management.
With the mobile station having the above-described constitution, when the above-mentioned time management is performed, the system time counter, on which the time management is based, always follows a reference signal. Under a severe communication environment such as fading, however, as shown in FIG. 3, assuming that a search window ranges from 44 to 84 in the phase and noise E whose correlation value reaches a prescribed level or higher exists at a phase position 45 in the end part of the xe2x80x9csearch windowxe2x80x9d, the mobile station erroneously detects the noise E, which has the smallest phase, as a reference signal.
In this case, since the search window range for the steady search to be performed next by the mobile station is changed to a search window range between xc2x120 of the phase position 45 of a xe2x80x9cfalse reference signalxe2x80x9d positioned at the center as shown in FIG. 4, the multipass signals S2 and S3 at the phase positions 68 and 70 cannot be detected and only the multipass signal S1 located at the phase position 64 can be detected.
However, under an actual fading environment, the multipass signal S1 can not be often detected because of its signal strength falling by 30 dB or more. If this phenomenon occurs during the steady search, in the worst case, the multipass signal S1 to be naturally received cannot be disadvantageously detected again, so that the reference signal has lost.
Further, the noise E which is accidentally erroneously detected is detected again with an extremely low probability during a next steady search, so that the reference signal, on which the time management depends, is completely lost. In this instance, as illustrated in FIG. 5, the search window actually begins to shift gently either forward or backward by the clock error of the system time counter, which is employed by the base station and the mobile station, with the phase position 45 as the center, and the steady search is performed within the then shifted phase range.
If the shift direction is a direction illustrated by an arrow mark, the search window range further shifts from the phase range of xc2x120 of the phase position 45 of the xe2x80x9cfalse reference signalxe2x80x9d positioned at the center toward a direction in which the multipass signals S1 to S3 cannot be detected. Therefore, also in this case, the multipass signal S1 cannot be detected again and thus, the reference signal has lost.
In view of the foregoing, and object of this invention is to provide a receiving method and receiver for spread spectrum signals in which delay timings of delay signals received through a plurality of transmission paths are detected in a short time and the delay signals are accurately demodulated.
The foregoing object and other objects of the invention have been achieved by the provision of a receiving method for spread spectrum signals, in which the spread spectrum signals transmitted by spectrum-diffusing a modulation signal are received through a plurality of transmission paths as a plurality of spread spectrum signals, the plurality of spread spectrum signals are inverse-spectrum-spread by using diffusion codes having different phases respectively corresponding to the receiving timings of the plurality of spread spectrum signals and synthesized while setting their timings the same in order to generate a demodulation signal. In addition, receiving timing of a spread spectrum signal having the largest correlation value, out of the received plurality of spread spectrum signals, is used as a reference phase position to decide a predetermined detection phase range. Then, the spread spectrum signals of which correlation values with the respective diffusion codes is a predetermined level or higher are detected within the predetermined detection phase range, in order to generate a demodulation signal from the spread spectrum signals, of which the correlation values with the respective diffusion codes are the predetermined level or higher. Afterwards, the steps of setting the predetermined detection phase range, detecting the spread spectrum signals, and generating a demodulation signal are repeated. If the spread spectrum signals, of which the correlation values with the respective diffusion codes are the predetermined level or higher, are not detected within the predetermined detection phase range, the detection phase range is widen based on the reference phase position. Therefore, even the case where noises or the like cause movement of the detection phase range and a plurality of delay signals to be originally detected are lost, the delay timings can be simply detected only by searching the predetermined detection phase range further widen. Thus, a plurality of delay signals are demodulated according to the respective delay timings.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.